Numerous processes and catalysts are known in the prior art for the production of unsaturated, carboxylic acids by the oxidation of unsaturated aldehydes, e.g., the production of acrylic acid from acrolein. A very efficacious process of this type is shown in U.S. Pat. No. 3,644,509 which discloses producing an unsaturated carboxylic acid, such as acrylic acid, by reacting the corresponding unsaturated aldehyde, such as acrolein, with oxygen in the presence of a catalyst having the empirical formula Mo.sub.a V.sub.b W.sub.c Mn.sub.d O.sub.e, the atomic ratio of Mo:V:W:Mn:O being such that when a is 12, b is 0.5 to 12, c is 0.1 to 6, d is 0.5 to 20 and e is 37 to 94.
The search has continued for improved catalysts and processes to produce unsaturated carboxylic acids by the oxidation of unsaturated aldehydes.
Metal oxide catalysts are often used "neat" (i.e, unsupported) in processes of this type and often as particulates in a bed or zone within a reaction vessel. Generally, the reactants are introduced, either separately or concurrently, at one end of the zone and flow co-currently through the reaction zone in contact with the particular catalytic material therein with the reaction product stream (including the desired product) exiting from the zone at the end opposite introduction.
A number of difficulties have been found to arise in the operation of such neat metal oxide-catalyzed processes. It has been found, for example, that substantially all (i.e, more than 75 percent) of the total conversion occurs in the first portion of the catalyst zone or bed (i.e., the first 25 percent of the total catalyst zone or bed) which is in contact with the reactants. The concentration of conversion in the first portion of the zone in an exothermic reaction raises the exotherm temperature at that point substantially in excess of that at later points in the catalyst zone. These high exotherm temperatures make control of the reaction more difficult, dictate more expensive heat-resistant materials, often decrease the yield of desired product and/or increase the yield of undesired by-products and are otherwise disadvantageous.
Generally, attempts to solve the problems associated with such a concentration of conversion in the initial portion of a neat catalyst zone have focused on diluting the catalyst in such a manner so as to essentially homogenize the exotherm temperature throughout the length of the catalyst zone. Dilution has been attempted both by adding separate discrete particles of a non-catalytic material uniformly dispersed with the catalyst particles and by mixing the catalyst material with the non-catalytic material (in solution, slurry or the like) and forming the resulting mixture into relatively homogenous particles of an admixture of catalytic and non-catalytic material for use in the catalytic reaction zone.
These types of dilution of catalytic material in a reaction zone have been found, however, to manifest other problems. That is, it is frequently considerably more expensive to prepare the diluted catalyst zones or beds. In addition, product yields from such diluted catalyst zones are often lower both in total product yield and product selectivity than with the corresponding neat catalyst zones.